The redundant sex
that results after several years of marriage
. It is not necessarily bad, just boring. You know she likes "this" and she knows you like "that" so all you do is "this and that". There tends to be no deviation from the set pattern after several years and all foreplay disappears. Undress, lie down, hump (only 1-2 positions allowable), and done.
Me: "I wanted to 69 with the wife last night and get out the ropes but all I got was married sex."
Friend: "Bummer, man. That must have been boring!"